FIG. 1 shows a superhard material 100 that is insertable within a downhole tool (not shown), such as a drill bit or a reamer, in accordance with an exemplary embodiment of the invention. One example of a superhard material 100 is a cutting element 100, or cutter or insert, for rock bits, as shown in FIG. 1. However, the superhard material 100 can be formed into other structures based upon the application that it is to be used in. In other examples, the superhard material 100 is a rock sample, which can be obtained from within a wellbore or from other sources. The cutting element 100 typically includes a substrate 110 having a contact face 115 and a cutting table 120. The cutting table 120 is fabricated using an ultra hard layer which is bonded to the contact face 115 by a sintering process according to one example. According to some examples, the substrate 110 is generally made from tungsten carbide-cobalt, or tungsten carbide, while the cutting table 120 is formed using a polycrystalline ultra hard material layer, such as polycrystalline diamond (“PCD”) or polycrystalline cubic boron nitride (“PCBN”). These cutting elements 100 are fabricated according to processes and materials known to persons having ordinary skill in the art. Although the cutting table 120 is shown having a substantially planar outer surface, the cutting table 120 can have alternative shaped outer surfaces, such as dome-shaped, concave-shaped, or other non-planar shaped outer surfaces, in other embodiments. Although some exemplary formulations for the cutting element 100 have been provided, other formulations and structures known to people having ordinary skill in the art can be used depending upon the application. Although rock drilling is one application that the superhard material 100 can be used in or obtained from and which is described hereinbelow, the superhard material 100 can be used in or obtained from various other applications including, but not limited to, machining, woodworking, and quarrying.
Different PCD, PCBN, hard, and superhard material grades are available for the cutters 100 to be used in various applications, such as drilling different rock formations using different drill bit designs or machining different metals or materials. Common problems associated with these cutters 100 include chipping, spalling, partial fracturing, cracking, and/or flaking of the cutting table 120 during use. These problems result in the early failure of the cutting table 120 and/or the substrate 110. Typically, high magnitude stresses generated on the cutting table 120 at the region where the cutting table 120 makes contact with earthen formations during drilling can cause these problems. These problems increase the cost of drilling due to costs associated with repair, production downtime, and labor costs. Thus, an end-user, such as a bit designer or a field application engineer, chooses the best performing grade of the cutter 100 for any given drilling or machining task to reduce these common problems from occurring. For example, the end-user selects an appropriate cutter 100 by balancing the wear resistance and the impact resistance of the cutter 100, as determined using conventional methods. Typically, the information available to the end-user for selecting the appropriate grade cutter 100 for a particular application is derived from historical data records that show performance of different grades of PCD, PCBN, hard, or superhard material in specific areas and/or from laboratory functional tests which attempt to mimic various drilling or machining conditions while testing different cutters 100. There are currently two main categories of laboratory functional testing that are used in the drilling industry. These tests are the wear abrasion test and the impact test.
Superhard materials 100, which include polycrystalline diamond compact (“PDC”) cutters 100, have been tested for abrasive wear resistance through the use of two conventional testing methods. The PDC cutter 100 includes the cutting table 120 fabricated from PCD. FIG. 2 shows a lathe 200 for testing abrasive wear resistance using a conventional granite log test. Although one exemplary apparatus configuration for the lathe 200 is provided, other apparatus configurations known to people having ordinary skill in the art can be used without departing from the scope and spirit of the exemplary embodiment.
Referring to FIG. 2, the lathe 200 includes a chuck 210, a tailstock 220, and a tool post 230 positioned between the chuck 210 and the tailstock 220. A target cylinder 250 has a first end 252, a second end 254, and a sidewall 258 extending from the first end 252 to the second end 254. According to the conventional granite log test, sidewall 258 is an exposed surface 259 which makes contact with the superhard component 100 during the test. The first end 252 is coupled to the chuck 210, while the second end 254 is coupled to the tailstock 220. The chuck 210 is configured to rotate, thereby causing the target cylinder 250 to also rotate along a central axis 256 of the target cylinder 250. The tailstock 220 is configured to hold the second end 254 in place while the target cylinder 250 rotates. The target cylinder 250 is fabricated from a single uniform material, which is typically granite. However, other rock types have been used for the target cylinder 250, which includes, but is not limited to, Jackfork sandstone, Indiana limestone, Berea sandstone, Carthage marble, Champlain black marble, Berkley granite, Sierra white granite, Texas pink granite, and Georgia gray granite.
The PDC cutter 100 is fitted to the lathe's tool post 230 so that the PDC cutter 100 makes contact with the target cylinder's 250 exposed surface 259. The lathe's tool post 230 draws the PDC cutter 100 back and forth across the exposed surface 259. The tool post 230 has an inward feed rate on the target cylinder 250. The abrasive wear resistance for the PDC cutter 100 is determined as a wear ratio, which is defined as the volume of target cylinder 250 that is removed to the volume of the PDC cutter 100 that is removed. Alternatively, instead of measuring volume, the distance that the PDC cutter 100 travels across the target cylinder 250 can be measured and used to quantify the abrasive wear resistance for the PDC cutter 100. Alternatively, other methods known to persons having ordinary skill in the art can be used to determine the wear resistance using the granite log test. Operation and construction of the lathe 200 is known to people having ordinary skill in the art. Descriptions of this type of test is found in the Eaton, B. A., Bower, Jr., A. B., and Martis, J. A. “Manufactured Diamond Cutters Used In Drilling Bits.” Journal of Petroleum Technology, May 1975, 543-551. Society of Petroleum Engineers paper 5074-PA, which was published in the Journal of Petroleum Technology in May 1975, and also found in Maurer, William C., Advanced Drilling Techniques, Chapter 22, The Petroleum Publishing Company, 1980, pp. 541-591, which is incorporated by reference herein.
FIG. 3 shows a vertical boring mill 300 for testing abrasive wear resistance using a vertical boring mill (“VBM”) test or vertical turret lathe (“VTL”) test. Although one exemplary apparatus configuration for the VBM 300 is provided, other apparatus configurations can be used without departing from the scope and spirit of the exemplary embodiment. The vertical boring mill 300 includes a rotating table 310 and a tool holder 320 positioned above the rotating table 310. A target cylinder 350 has a first end 352, a second end 354, and a sidewall 358 extending from the first end 352 to the second end 354. According to the conventional VBM test, second end 354 is an exposed surface 359 which makes contact with a superhard material 100 during the test. The target cylinder 350 is typically about thirty inches to about sixty inches in diameter; however, this diameter can be greater or smaller.
The first end 352 is mounted on the lower rotating table 310 of the VBM 300, thereby having the exposed surface 359 face the tool holder 320. The PDC cutter 100 is mounted in the tool holder 320 above the target cylinder's exposed surface 359 and makes contact with the exposed surface 359. The target cylinder 350 is rotated as the tool holder 320 cycles the PDC cutter 100 from about the center of the target cylinder's exposed surface 359 out to about its edge and back again to about the center of the target cylinder's exposed surface 359. The tool holder 320 has a predetermined downward feed rate. The VBM method allows for higher loads to be placed on the PDC cutter 100 and the larger target cylinder 350 provides for a greater rock volume for the PDC cutter 100 to act on. The target cylinder 350 is typically fabricated from granite; however, the target cylinder can be fabricated from other materials that include, but is not limited to, Jackfork sandstone, Indiana limestone, Berea sandstone, Carthage marble, Champlain black marble, Berkley granite, Sierra white granite, Texas pink granite, and Georgia gray granite.
The abrasive wear resistance for the PDC cutter 100 is determined as a wear ratio, which is defined as the volume of target cylinder 350 that is removed to the volume of the PDC cutter 100 that is removed. Alternatively, instead of measuring volume, the distance that the PDC cutter 100 travels across the target cylinder 350 can be measured and used to quantify the abrasive wear resistance for the PDC cutter 100. Alternatively, other methods known to persons having ordinary skill in the art can be used to determine the wear resistance using the VBM test. Operation and construction of the VBM 300 is known to people having ordinary skill in the art. A description for this type of testing can be found in Bertagnolli, Ken and Vale, Roger, “Understanding and Controlling Residual Stresses in Thick Polycrystalline Diamond Cutters for Enhanced Durability,” US Synthetic Corporation, 2000, which is incorporated by reference in its entirety herein.
In addition to testing for abrasive wear resistance, PDC cutters 100 also can be tested for resistance to impact loading. FIG. 4 shows a drop tower apparatus 400 for testing impact resistance of superhard components using a “drop hammer” test where a metal weight 450 is suspended above and dropped onto the cutter 100. The “drop hammer” test attempts to emulate the type of loading that can be encountered when the PDC cutter 100 transitions from one formation to another or experiences lateral and axial vibrations. Results from the impact testing allows for ranking different cutters based upon their impact strength; however, these ranking do not allow for predictions to be made according to how the cutters 100 will perform in the actual field.
Referring to FIG. 4, the drop tower apparatus 400 includes a superhard material 100, such as a PDC cutter, a target fixture 420, and a strike plate 450 positioned above the superhard material 100. The PDC cutter 100 is locked into the target fixture 420. The strike plate 450, or weight, is typically fabricated from steel and is positioned above the PDC cutter 100. However, the strike plate 450 can be fabricated from alternative materials known to persons having ordinary skill in the art. The PDC cutter 100 is typically held at a backrake angle 415 with the diamond table 120 of the PDC cutter 100 angled upward towards the strike plate 450. The range for the backrake angle 415 is known to people having ordinary skill in the art.
The strike plate 450 is repeatedly dropped down on the edge of the PDC cutter 100 until the edge of the PDC cutter 100 breaks away or spalls off. These tests are also referred to as “side impact” tests because the strike plate 450 impacts an exposed edge of the diamond table 120. Failures typically appear in either the diamond table 120 or at the contact face 115 between the diamond table 120 and the carbide substrate 110. The “drop hammer” test is very sensitive to the edge geometry of the diamond table 120. If the table 120 is slightly chamfered, the test results can be altered considerably. The total energy, expressed in Joules, expended to make the initial fracture in the diamond table 120 is recorded. For more highly impact resistant cutters 100, the strike plate 450 can be dropped according to a preset plan from increasing heights to impart greater impact energy on the cutter 100 to achieve failure. However, this “drop hammer” test embodies drawbacks in that this method requires that many cutters 100 be tested to achieve a valid statistical sampling that can compare the relative impact resistance of one cutter type to another cutter type. The test is inadequate in providing results that reflect the true impact resistance of the entire cutter 100 as it would see impact loads in a downhole environment. The test exhibits a static impact effect whereas the true impact is dynamic. The number of impacts per second can be as high as 100 hertz (“Hz”). Also, the amount of damage to the cutter 100 is subjectively evaluated by someone with a trained eye and is compared to damages incurred by other cutters.
While the results for different wear tests available in the market have generally a reasonable degree of agreement with the actual field performance, the same is not the case for the results of conventional impact tests. Although there is some degree of correlation between the results of conventional impact tests and actual field performance, the scattering of the data is usually very large, thereby causing predictions on how cutters will behave in actual field performance to be difficult and/or inaccurate. Also, many fractures occurring within the cutter are not detected using these conventional tests and therefore go undetected when evaluating the toughness of the cutter.
Additionally, since the bit selection is a critical process, it is important to know the mechanical properties of the different rocks, or rock formations, the bit is to drill through. One of the most important parameters currently used for the bit selection is the unconfined compressive strength (“UCS”) of the rock, which can be measured directly on core samples obtained from the rock formations within the wellbore or evaluated indirectly from log data. However, the UCS of the rock should not be solely relied on when selecting the bit because the UCS can be misleading, especially when the rock UCS is greater than 15000 psi and is brittle, thereby having a low fracture toughness K1C. Thus, fracture toughness of the rock should also be considered when selecting the proper drill bit.
In oil well drilling and completion operations, it is beneficial to know the compressive strength of various rock formations within the well plan. These compressive strengths can be used to plan drilling operations since, as previously mentioned, the compressive strength affects the drill bit selection and casing plan among other drilling operations. For completions operations, it is beneficial to know the compressive strength to plan fracturing operations. One method used for determining the compressive strength of a rock formation within the wellbore is to calculate the compressive strength from electric log data, such as sonic logs, electric resistivity, and gamma logs. However, inaccuracies occur when calculating compressive strength from electric log data. Inaccuracies develop because log interpretation requires some calibration with core data. Another method used for determining the compressive strength of a rock formation within the wellbore is to retrieve a core sample, or rock sample, from the rock formation within the wellbore and directly measure the compressive strength. However, inaccuracies also occur when directly measuring the compressive strength from a retrieved core sample. Inaccuracies develop because the calculated compressive strength is performed at the surface and not in the wellbore; and therefore, it is the unconfined compressive strength (“UCS”) that is determined rather than the confined compressive strength (“CCS”).
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.